server: accept image data as multipart request

[adamdecaf]

Related to https://github.com/moov-io/imagecashletter/issues/86 we should also accept image data as mult-part form params which can be replaced in the JSON representation. Bundling base64'd image data blobs in JSON can be a pain. The previous format would still be accepted.

I'm thinking that ImageViewData's ImageData could have identifiers related to the form blocks.

The block containing JSON would be named file.

{
  // ... 
  "imageViewData": {
    "imageData": ":checkOne:"
  },
  // ... 
}

Then a second block/boundary called checkOne with the encoded image data. The HTTP server would replace that data directly into the ImageViewData struct. We could try base64 decoding this data.